Patient Guide to Genetic Testing

Palo Alto, CA – What is genetic testing?

Genetic testing allows physicians and researchers to identify variations in a patient’s DNA to determine if an abnormal gene is present and causing disease and if there is a chance that the DNA variance can be passed on to the patient’s descendants.

Patients who come in for a genetic test usually undergo a full clinical exam with Dr. Vinit Mahajan, Director of the Molecular Surgery Program. Patients then agree to give a small sample of blood or saliva for analysis. Usually, a blood draw is performed by a nurse during the patient’s appointment or with a Clinical Research Coordinator. 

Genetic test results usually take several months, and patients review the results with Dr. Mahajan or a genetic counselor. 

What are the different types of genetic tests? 

There are approximately 20,000 different human genes coding for different proteins.

Genetic “panel” tests are performed on a patient’s DNA to look for disease-causing DNA abnormalities in the nearly 300 genes known to cause eye disease. At Stanford, Dr. Mahajan participates in the Foundation Fighting Blindness gene testing program. It is estimated that researchers know over half of the genes that cause retinal dystrophy and retinitis pigmentosa. 

When gene panel testing doesn’t find a disease-causing DNA variant, some patients will undergo “whole exome” or “whole genome” genetic testing. These tests look at all the genes that code for proteins or the entire human DNA sequence, respectively.

While the gene tests are commonly done on short lengths of DNA, “chromosomal” genetic tests are done on long lengths of DNA to see if there are large genetic changes, like a multigene deletion. 

Biochemical genetic tests study the amount or activity level of gene products, such as proteins.

Who should get genetic testing?

Individuals who have a family history of certain types of inherited eye problems, including retinitis pigmentosa, macular degeneration, vitreous degenerations, uveitis, cataracts, and glaucoma, for example, are good candidates for genetic testing. Some patients without a family history can also be good candidates, depending on their clinical exam.

Individuals with family members who tested positive for a genetic mutation may also benefit from testing.

Individuals who are considering genetic testing at Stanford’s Byers Eye Institute will meet with a Clinical Research Coordinator, such as Teja Chemudupati, sometime during their clinical appointment. Patients will have a chance to discuss the pros and cons of genetic testing, the testing process, what to expect from genetic testing results, and where they can find more information. Patients must give permission following an informed consent discussion regarding genetic testing in order to proceed.

Who should not get genetic testing?

Not everyone should get genetic testing. The decision to get tested should be based on a person’s personal and family histories and a clinical exam.

Physicians advise against testing children under 18, because treatments are often not available, and the results can negatively impact a child’s life. 

Testing can provide a sense of relief for a patient, but it may also trigger fear, anxiety or guilt, especially because doctors can’t always tell a person with a mutation what their risk is of developing a disease. Genetic markers can indicate if you are at above-average risk for contracting a disease, but it doesn’t mean that you will actually contract the disease. And the absence of genetic risk markers don’t alwaysensure that you won’t get a disease.

Many of the risks associated with genetic testing involve the emotional, social, or financial consequences of the test results.

Working with a doctor and genetic counselors is important to address the issues that go along with genetic testing.

How are genetic testing results used in the clinic?

Genetic testing can guide treatment and make patients eligible for clinical trials that offer therapies not yet on the market. Knowing the gene is the first step in that process. Having a genetic test result can be beneficial, since it can allow patients as well as healthcare providers to search for any clinical trials that the patient may qualify for given their confirmed genotype. Stanford is participating in gene therapy studies.

Genetic testing can also help catch diseases at their earliest stages, allowing personalized medicine for better monitoring and treatment plans. 

How are genetic testing results used in the laboratory?

Molecular surgery, gene editing, and drug repositioning have all come out of laboratory research conducted on tissues and fluids collected from patients. Patient-provided blood samples, as well as other types of samples, are banked in the Eye Institute’s research biorepository. Stanford’s biorepository houses tissues from patients with genetic mutations, so that they can be used by scientists to develop new therapies in the laboratory.

Does insurance cover genetic testing?

Some health insurance providers may pay for genetic testing, but not all genetic testing is covered. Patients who are considering genetic testing should discuss coverage with their providers prior to their genetic testing appointment. In some cases, a patient may have genetic testing provided at little or no cost. 

Where can you get a genetic test?

Genetic tests are offered at Stanford’s Byers Eye Institute with Dr. Mahajan and other medical research centers across the US. For a clinical appointment, you or your doctor can contact us.

There are companies that provide over-the-counter saliva DNA tests they claim can reveal the likelihood that a person will develop a disease. These tests are considered controversial due to the uncertainty of results, and physicians do not routinely recommend them.

What is gene therapy?

Gene therapy is an experimental technique that uses genes to treat or prevent disease. In the future, this technique may allow doctors to treat a disorder by inserting a gene into a patient's cells instead of using drugs or surgery. Dr. Mahajan was a surgeon for the landmark gene therapy trial for RPE65 that was recently approved by the FDA.